1. Field of the Invention
The present invention relates to a mandrel used for manufacturing hollow portions such as a hollow rack bar, etc. used for a power steering system of a car, or the like, a set of mandrels, and a hollow rack bar manufactured using the mandrel.
2. Description of the Related Art
The related arts have manufactured a hollow rack bar (hollow portion) used for a power steering system of a car by cutting a round bar. However, in order to meet a sophisticated shape and lightweight, a method of manufacturing the hollow rack bar by performing a transfer forging process on a pipe has well been known (for example, see Jpn. Pat. Appln. KOKOKU Publication No. 3-5892). Specifically, the pipe is first pressed by a hot forging die so that it is primarily molded in a tooth shape and at the same time, its upper surface is flatly formed and then, a mandrel is pressed into a cavity of the pipe. The mandrel has projected portions in a taper shape. The projected portions are connected with an inner peripheral side of the flat portion of the pipe so that the wall of the flat portion is overhung by flowing toward teeth of a mold frame in a plastic deformation way and an outer peripheral side of the flat portion of the pipe is provided with teeth in a straight line direction by a transfer method, the teeth in a straight line direction having a shape corresponding to the teeth of the mold frame, making it possible to manufacture the rack bar.
It has been known that the extent Ta to Tc of projected portions 204 to 206 mounted from a leading end 202 of a bar 201 of a mandrel 200 toward a base end 203 thereof is formed to be sequentially large from the pressed leading end 202 to the base end 203 as shown in FIG. 20 (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2006-026703).
The aforementioned mandrel has problems as follows. That is, since the extent Ta to Tc of the projected portions 204 to 206 gradually increases, a large load is not applied to the mandrel 200 from the pipe in the press direction. On the other hand, the mandrel 200 passes in the press direction and is then slightly returned by a springback so that an inner diameter of the pipe is small. Therefore, when the mandrel 200 passes in a withdrawing direction, the projected portion 206 with maximum extent first contacts an inner wall surface of the pipe so that large load is applied to the mandrel, thereby applying excessive pressure to the mandrel 200. As a result, the case where the lifetime of the mandrel is shortened has become frequent.
Meanwhile, in order to prevent excessive load from being applied to the mandrel, if the extent of the projected portion 204 with the maximum extent of the mandrel 200 is small, the deformation due to the forging process per one time is reduced, thereby causing problems that the number of work processes is increased, kinds of required mandrels are increased, and processing efficiency is degraded.
Further, the rack bar is used for a rack and pinion gear 300 shown in FIG. 21. The rack and pinion gear 300 includes a casing 301, a pinion gear 303 rotatably mounted to the casing 301 through a bearing 302, a hollow rack bar 304 movably mounted to the pinion gear 303 in an orthogonal direction thereto, a rack guide 305 slidably supporting an outer wall surface (angle range M) of the hollow rack bar 304, and a spring 306 pressing the rack guide 305 to the pinion gear 303.
FIG. 22 is a cross-sectional view showing one example of a mandrel 400. The mandrel 400 includes a bar 401 and a projected portion 402 mounted to the bar 401. Also, a cross section of the bar 401 is a vertically long shape as shown in FIG. 22.
The aforementioned mandrel has problems as follows. That is, an outer peripheral surface of the hollow rack bar 304 should be formed at high precision so as to smooth the sliding of the rack guide 305.
To the contrary, a rear surface side of the projected portion 402 of the mandrel 400 is formed to have only a narrow angle range based on an axial direction of the mandrel 400. Therefore, effort and time to perform the processing, such as formation of the hollow rack bar 304 as an R shape in a subsequent process, is required.
Further, there is a case where the transfer forging is performed using a short type mandrel instead of a long type mandrel. Such a short type mandrel includes a separate driving bar so as to reciprocate the mandrel. In this case, the mandrel may be collapsed within the hollow portion of the hollow rack bar 304, making it impossible to mold the tooth.
Meanwhile, there is a problem that the lifetime of the mold frame used when transferring the teeth is shortened since stress is intensively applied to a center portion of the tooth bottom when transferring.
Also, such a mandrel is configured of a plurality of sets in which the extent of the projected portion gradually increases, and uses an optimal combination of the diameter, the material, the shape of a mold frame, etc. of the hollow rack bar.
The set of mandrels has problems as follows. That is, reaction force applied from the pipe to the mandrel makes the projected extent large. That is, since the projected extent becomes large on reaching the end of the processing, if the maximum projected extent of the next mandrel is set to be excessively large as compared to the maximum projected extent of a previous mandrel, a large load is required when pressing and the mandrel cannot be pressed due to an excess of capacity of a driving source.
Further, the mandrel passes in the pressing direction and is then slightly returned by the springback so that the inner diameter of the pipe is small. Therefore, when a next mandrel is pressed, the projected portion with the maximum extent first contacts the inner wall surface of the pipe so that large load is applied to the mandrel, thereby applying the excessive pressure to the mandrel. As a result, the case where the lifetime of the mandrel is shortened has become frequent.
Also, there is a case where the inner diameter of the pipe is small due to the occurrence of abnormality for any reasons when forming the flat portion. In this case, when a general mandrel is pressed, the mandrel is damaged.
A mandrel 300 used for the transfer forging includes a bar 301 and a projected portion 302 projected from the bar 301 as shown in FIG. 23. Also, the mandrel 300 is configured of a plurality of sets in which the extent of the projected portion 302 gradually increases as shown in a two-dot chain line in FIG. 23 and uses an optimal combination of the diameter, the material, the shape of the mold frame, etc. of the hollow rack bar.
The mandrel has problems as follows. That is, a maximum width W of the bar 301 of the mandrel 300 is identically set, irrespective of the extent of the projected portion 302. As a result, as the extent of the projected portion 302 becomes large, the molding load tends to be increased by pressing or sliding with respect to a predetermined width S1 of the inner wall surface in the transverse direction of the pipe P, as shown in FIG. 24. Also, if the width of the bar fluctuates due to an error in a manufacturing step, the molding load also fluctuates.
Therefore, there is a need for an apparatus generating large molding load in consideration of the fluctuation of molding load and there is a risk of increasing the cost of equipment.